I understand why you are looking for a side-by-side component system mattress as this will allow you to customize each side for the weight differential between you and your partner so that it is suitable for both of you. Also, this layered configuration will allow you to adjust and change firmness or softness as needs change over time. (As a manufacturer of systems like these I am very familiar with these designs ) For example, if you start with a certain setup and the comfort and support is appropriate for you but your partner is still experiencing back aches, you can unzip the cover and rearrange the layers to get more support and a firmer feel.
From the information you provided I am still not able to determine if you are looking for a Memory Foam comfort layer in your mattress or what the budget range is but usually Memory Foams have ILDs in the low numbers which means that they provide too much plushness for your desired needs. I am not sure if you are stuck on the idea of a Memory Foam top layer but if you like to feel of latex (which has more resilience and support but is still very good at motion isolation) you would have more of a chance to get the firm or extra firm feel you are after in a new mattress.
If you like the feel of memory foam for the top of the mattress, I’d recommend at least a 5 lb foam because, at your weight, you want to make sure that memory foam layer is going to last for a while. If you are considering an all-latex mattress, I would suggest that your partner uses a 3"of medium over 3" of medium over 3" of firm for your partner’s mattress. If she tries that and finds that she’s still getting lower back aches, she can move the firm layer to the middle for medium/firm/medium. If that’s still too soft, she can try firm/medium/medium, and if that’s still to soft, please call us and we’ll send another firm or even extra firm layer for $45. For your side of the mattress, I would recommend medium over firm over firm. If you try this and find that it’s too soft, you can try firm/medium/firm or even firm/firm/medium. Please keep in mind that in making these recommendations
I am using one of the systems that SleepEZ is offering (you can see an example here) that fits your description of the mattress you want and this recommendation may not work the same way for other mattresses that have a similar construction but different materials. Our initial recommendations have more than 90% success rate which means that there’s a 90% chance that the above recommendation would work perfectly for the two of you. However, if after some in-home testing and after allowing your body to readjust to the new sleeping surface (which can take anywhere between 1 and 4 weeks especially if you come from a Memory Foam feel type of surface) you find that the comfort or support level is not appropriate you have to make sure that you chose something with a good return/exchange policy to allow for further customization, For example in cases like this we ship a layer exchange for a flat-rate $45 fee for the exchange which can include two 36" x 84" layers).
Also, since latex mattresses typically last 20 years, if anything changes in 5, 10, 15, or even 20 years (if you gain weight, lose weight, develop hip or shoulder issues, etc. etc.) your mattress can always change along with you. So, instead of buying a new mattress, you’re really just looking at rearranging the layers or buying a single layer instead of buying an entirely new mattress.
Layered mattresses are especially recommended for people with high BMI ranges because, if the mattress starts to break down in 10 or 15 years, you would only have to buy a new top layer instead of a new mattress, so it makes it much more affordable and cost effective in the long run.
I hope this information helps you, but if I can be of any additional assistance, please just let me know!